The invention relates to a radial piston pump, with cylinders oriented radially to an axis of rotation of an eccentric shaft, and with pistons arranged radially movably in the cylinders against the force of a spring. The pistons are pressed radially outwards by the rotational movement of an eccentric and are pressed radially inwards by the spring. The pistons have an inlet opening connected to an inlet chamber of a pumping medium when the pistons are in the radially inner positions. The pumping medium is pressed into a pressure area during the radially outward movement of the pistons. The eccentric shaft is mounted in sliding bearings arranged on both sides of the eccentric and is drivable by a traction means.
Radial piston pumps of this type are known. The alternating radial inward and outward movements of the pistons in the cylinders pump a medium, for example oil, is conveyed in a known manner. Radial piston pumps of this type are used for levelling systems in motor vehicles for example. In that case, the radial piston pump is driven by a belt drive which is driven by an internal-combustion engine of the motor vehicle. The belt engages on a drive wheel of the radial piston pump in order to rotate the eccentric shaft of the radial piston pump. The arrangement of the radial piston pump applies a belt force having a radial direction vector upon the eccentric shaft by the belt drive. The direction vector and the amount of the belt force are substantially constant.
In addition, the eccentric shaft is loaded by hydraulic forces which are introduced by the pistons of the radial piston pump and which likewise have a radial direction vector. A resulting hydraulic force of the radial piston pump, formed from partial hydraulic forces, is produced in accordance with the number of pistons of the radial piston pump. In this case the level and the direction vector of the resulting hydraulic force vary during use of the radial piston pump for its intended purpose in accordance with a rotational speed of the eccentric shaft. The constant belt force is overlaid by the variable hydraulic force, causing the eccentric shaft to be acted upon with a varying radial force. The resulting hydraulic force (also referred to as the "bearing force" below) has to be removed by the sliding bearings in which the eccentric shaft is mounted.
With large volumes in the radial piston pump and high hydraulic pressures, the resulting hydraulic forces can have a greater total than the belt force and, depending upon their operative direction, the hydraulic forces can cause a change in direction of the resulting force acting upon the eccentric shaft. In this way, the eccentric shaft can be pressed onto the sliding bearing against the belt force by the hydraulic forces. In this case, the actual resulting hydraulic force determines the direction vector of the resulting bearing force of the eccentric shaft and thus specifies a position of the eccentric shaft in the sliding bearing.
A drawback of this is that the change in position of the eccentric shaft in the sliding bearings can generate noise, a so-called knocking, as well as increased wear. In particular, if the radial piston pump is suction-throttled and is operated heavily regulated, phases can occur in which none of the pistons of the radial piston pump conveys the pumping medium, so that the eccentric shaft is oriented exclusively by the belt force as a result of the absence of hydraulic forces. At the beginning and the end of this phase, the resulting bearing force changes abruptly with respect to its direction vector, so that a reciprocating movement of the eccentric shaft occurs in the sliding bearings.
In addition, the hydraulic force acting upon the eccentric shaft does not change continuously, but changes abruptly, with respect to both the amount and the direction vector. Depending upon whether a piston of the radial piston pump begins or ceases to convey, the hydraulic force and thus the resulting bearing force produced by the superimposition with the belt force suddenly change.
It is known to lubricate the sliding bearings of the eccentric shaft in radial piston pumps with the pumping medium, for example oil. This oil is generally heavily foamed, particularly in the case of suction-regulated radial piston pumps, so that mixed friction of the eccentric shaft in the sliding bearings occurs as a result of air inclusions in the pumping medium. The mixed friction is not sufficient to damp the above-mentioned knocking of the eccentric shaft in the sliding bearings.